For the purpose of detection of an obstacle present in periphery of a vehicle, both image information and distance information are important. Image information can provide distinct information for obstacle detection through driver's visual perception. Distance information is useful in determination of whether an object is located at a position where it can be potentially be an obstacle for the vehicle. The image of the vehicle's periphery is obtained by a camera mounted on the vehicle. The peripheral image thus obtained will be displayed, as it substantially is, on a display unit mounted inside the vehicle or put for use in various image recognition schemes. The distance to the obstacle present in the vehicle's periphery can be alternatively obtained by calculation using an image processing technique. In such case, both image information and distance information can be obtained from image captured by the camera.
Patent Document 1 identified below discloses a technique for obtaining a perpendicular distance to an obstacle (object), with using two screens of differing view points. In this technique, a first screen and a second screen which have different view points from each other are generated by two cameras which are disposed in parallel on the right and left sides with a predetermined space therebetween. Then, the technique calculates an amount of displacement between corresponding object image portions of the respective image screens. Subsequently, by applying the triangulation principle to this displacement amount, the technique eventually obtains a perpendicular distance from a straight line joining the right and left cameras to the object.
According to the above-described object recognition technique, the technique requires precision alignment of the right and left cameras in order to obtain a stereoscopic view, since any misalignment between the right and left cameras will lead directly to an error in the position coordinates calculation of the object. Thus, if an apparatus implementing this object recognition technique is to be mounted on a motorcar or the like, there is not much freedom in the layout of the cameras on the motorcar. Therefore, there will be imposed high precision requirement on the mounting of the cameras.
In view of the above-described state of the art and others, for the purpose of determination of a distance to an obstacle located in periphery of a vehicle, there has often been employed a distance sensor exemplified by a sonar using supersonic wave. Patent Document 2 identified below shows a technique according to which cameras and a sonar are mounted and a display unit displays images obtained thereby with appropriate switchover in displaying condition of the images. More particularly, according to this technique, immediately after start-up of the vehicle, the display unit displays both an image of forward lateral side of the vehicle and an image of an area immediately ahead of and below the vehicle, in equal manner. Thereafter, if the sonar detects presence of an approaching obstacle ahead of the vehicle, the display unit will now display the image immediately ahead of and blow the vehicle with appropriate highlight, relative to the forward lateral side image. On the other hand, if the sensor detects no such obstacle, the display unit will display, this time, the forward lateral side image with appropriate highlight, relative to the image immediately ahead of and blow the vehicle.
In the case of this technique, the distance information obtained by the sonar is used in determination of whether the obstacle is located at a position wherein it can really be an obstacle for the vehicle. Whereas, the presence of the obstacle can be alarmed effectively as the image information to the driver. In this technique, however, the distance information comprises low resolution information indicative of only whether the obstacle is present at a position within a detection range of the sonar or not. As such, the shape characteristics of the obstacle remains unknown or it can be known, only with significant workload on the calculation by the image processing.
On the other hand, Patent Document 3 identified below discloses a technique relating to an apparatus for making a map of parking space in which a distance determining means such as a supersonic sensor and an image pickup means such an electronic camera respectively recognize a space available for vehicle parking. According to this technique, from an image obtained by the image pickup means, there is extracted a characteristic portion representing the depth of the image. Then, a distance to the extracted characteristic portion is calculated, based on a position on the obtained image and determination result of the distance determining means, whereby a space available for parking the vehicle is recognized as a rectangular space (see FIG. 5 of Patent Document 3), which then is made into and displayed as a parking space map.
Patent Document 1: Japanese Patent Application “Kokai” No. 5-114099 (paragraphs 16-20, FIGS. 3-5).
Patent Document 2: Japanese Patent Application “Kokai” No. 2004-142557 (paragraphs 5-7, FIGS. 1-3).
Patent Document 3: Japanese Patent Application “Kokai” No. 2002-170103 (paragraphs 3-12, paragraph 29, FIGS. 1-3 and FIG. 5).